In a conventional crystal oscillator circuit, in order to reduce the start-up time for the crystal in the circuit, an additional oscillator may be used as an excitation source. Before the crystal oscillator circuit starts to work, the frequency of the additional oscillator is calibrated to close to the frequency of the crystal oscillator circuit via a calibration circuit. In normal operation, the start-up of the crystal oscillator circuit is relatively fast after being excited by the oscillator.
However, if the excitation oscillator is a free-running oscillator, the frequency cannot change with the environmental temperature changes after the calibration. When temperature change is large, the frequency will deviate from the frequency of the crystal oscillator circuit too much and would not achieve the effect of excitation. Then the excitation oscillator needs to be recalibrated, which will increase the complexity of the circuit design.
As a result, a new method and circuit for exciting the crystal oscillation circuit may be necessary.